Topic: CB650 Idle mixture adjustment screw?

[Irishguy]

My 1979 CB650 runs great except for one little issue:

Right as you roll on the throttle there is a little hesitation.  It only occurs when you are transitioning from no throttle to a little throttle.  Just at the very start of the rotation.  It makes it a #$%* sometimes to start out at a stop light and makes you look like a total rookie sometimes.

Anyway I have searched and read on this subject and although I know that some here have replaced their main jets [and I may yet do that], I really don't want to tear into the carbs, because the bike runs so good otherwise.  I think what I need to do it richen up the idle mixture, but since I don't have a manual, I don't know where the screws are located.  So I have two questions:

1.  Where are the screws located?
2.  Which way do I turn them to richen up the mixture?

I plan on turning each one the same amount and I plan on only turning 1/4 and then taking a test ride on a fully warmed up bike each time until the condition becomes satisfactory.

Any help would be appreciated.  Thanks in advance.

[Irishguy]

OK...  I printed that section out of an online manual and it appears that the screws are on the bottom of the carbs in a little indentation in the float bowl.  [Is this correct?]  But it doesn't tell you which way to turn them.

[Buber]

to lean mixture - screw OUT. To richen - screw IN. The 2nd one is difficult... onthers are easy...

[Irishguy]

to lean mixture - screw OUT. To richen - screw IN. The 2nd one is difficult... onthers are easy...

Thanks man...

[Soos]

to lean mixture - screw OUT. To richen - screw IN. The 2nd one is difficult... onthers are easy...

I may be wrong here... but if I am rembering right the cb750 gets a richer idle circuit by screwing the idle screw in, and leaner by screwing out.(the idle adjuster is on the air filter side of the carbs)

And the cb650 having the idle screw adjustment on the other side of the carbs(the side towards the motor not the aircleaner) reverses the need to adjust the screw.
out to richen, in to lean it out.

please correct me if i am wrong here.

l8r

[Irishguy]

Well...  Great...  Now I have two different opinions.  I screwed all four of them in 1/4 of a turn, and I noticed no difference.  In fact it may be worse.  And to boot...  My hand is burned trying to adjust the screw on the number 2 cylinder carb.  That number 2 is a #$%* to get to. 

So...  Before I turn those screws again...  Which way is it to richen them up?  In or Out?

[Soos]

Well either way, I'd try the tested and true metheod of lightly seating them, and 1 5/8 turns out.

I just checked the still stock set of cb650('79) carbs I have, and they are 1 5/8 out.


You can always try going to the extremes and pull you choke to see the effect.

1st test :screw them all in lightly seated, and 1/4 turn out. then pull your choke once the bike is running, if the RPM increases, it's running lean. if it dies it's rich.

2nd test:try screwing them out... lets say 5 turns(or 1-2 turns before they fall out). then again try the choke. if it sputters and dies, it's rich. If it revs up wildly it's lean.



Sorry if I have confused you on this.... but i swear I'm right...


l8r


P.S. I'd do this test and reply on the results... but i'm running a set of cb750 carbs on my '79cb650.

[jtb]

Mark,

Out should be richer.

John

[333]

Understanding what these screws are may help in determining which way to rotate them.  I am not familiar with the 650s, and all my SOHC4s have been early ones, which are equiped with air adjuster screws, not gas adjuster screws.  I don't recall any bikes with gas screws, but I could be wrong.  I do have experience with different types of Honda small engines(outboard, generator,lawnmower as well as single,twin and four cyl Honda M/Cs) and Honda uses both in these different motors.  So...

Clockwise (in) closes the circuit (gas- leans the mix, air- richens the mix).
Counter Clockwise (out) opens the circuit (gas- richens the mix, air- leans the mix).

Have I totally confused the issue?

[dusterdude]

accelerator pump?

[Irishguy]

OK...  Thanks for all the suggestions.  It appears that I in fact did make the situation worse.

Soos' suggestion confirms what I have thought...  That my 650 is lean and my 500 is way rich.  The 650 will speed up the idle when I deploy the choke after the bike is warmed up and the 500, which I know is way rich will just die when you try to use the choke after it is warm.  As a matter of fact you do not even have to use the choke to start the 500.

Anyway...  I'm not sure at this point if I want to:

1.  Since I have apparently leaned them all out even more, just back them all of 3/4 of a turn from where I am and see how she runs. [This would probably be the safest option.]
2.  Screw them all in, and then back them all out 1 5/8".

Does anyone know what the factory spec is supposed to be?  I know that the original spec would be lean, but at least it would be a good reference point.

[Irishguy]

Well... After turning them all clockwise 1/4 turn the other day.  Today I turned them all counter clockwise 3/4 turn.  It now runs better and warms up quicker.  I think I will back them all off 1/4 of a turn tomorrow and take another test ride.

[eurban]

The 79 650s use slide type (non CV) accelerator pump type carbs that are similar to the 77/78 750 carbs.  The idle mix screws control the bleed of fuel so if you turn the screws out (counter clockwise) you will richen the mix.  You should make sure that your idle jets are truly clean before you fiddle around too much.  These jets are pressed in, but can be carefully removed, soaked in cleaner and then cleared with a single strand of copper wire.  As Duster mentions also make sure that your accelerator pump is working properly.  Take your air cleaner off, and with fuel in the bowls wack open the throttle.  You should see a squirt of fuel shoot towards the engine from each brass nozzle located on the bottom of the carb bore near the choke plate.  Set your idle mix screws to whatever the baseline setting is for the 79 650.  If problem still persists, try turning your idle screws out (or in) in 1/4 turn increments.  Good luck.